For best results, provide multiple high-resolution images with the following aspect ratios: 16×9, 4×3, and 1×1.

Quote from Google’s AMP with structured data

Since WPSSO Core and its add-ons already use a variety of image sizes for different markup standards (ie. Open Graph, Twitter Cards, Schema, etc.), it was fairly easy to add support for Google’s new Article AMP image sizes. You can find the new images sizes, along with all other WPSSO image sizes, under the WordPress Settings > SSO Image Sizes settings page.

The default (and recommended) image sizes are:

Open Graph (Facebook and Others)

1200x630px cropped.

Schema (Google and Pinterest)

1200x1800px uncropped.

Schema Article (Google and Pinterest)

1200x1800px uncropped.

Schema Article AMP 1:1 (Google)

1200x1200px cropped.

Schema Article AMP 4:3 (Google)

1200x900px cropped.

Schema Article AMP 16:9 (Google)

1200x675px cropped.

Schema Thumbnail Image

1200x630px cropped.

Twitter Summary Card

1200x630px cropped.

Twitter Large Image Summary Card

1200x1800px uncropped.

Note that several of the image sizes share common dimensions and crop settings — this allows WordPress to create fewer image files on disk (ie. only 5 image files for all 9 image sizes).